Semiconductor integrated circuit fabrication facilities (“fabs”) are highly automated. Movement of semiconductor wafers between various process tools is accomplished by an automated material handling system (AMHS). The wafers are typically transported through the fab in Front Opening Unified Pods (FOUPs), wafer holding devices capable of holding up to 25 wafers of 300 mm diameter.
A FOUP is a specialized enclosure designed to hold semiconductor wafers securely and safely in a controlled environment, and to allow the wafers to be removed for processing or measurement by tools equipped with appropriate load ports and robotic handling systems. Fins in the FOUP hold the wafers in place, and a front opening door allows robot handling mechanisms to access the wafers directly from the FOUP. A FOUP can be located on a load port, and can be manipulated by the AMHS.
The AMHS transport vehicles travel relatively long distances to carry the FOUPs between tools that perform different fabrication processes. The tools may be located within different portions of the same building, or in different buildings.
FIG. 1 is a diagram of a system 100. The system has a plurality of pieces of fabrication equipment (tools) 108a, 108b, which may be separated from each other by a relatively large distance (e.g., tens or hundreds of meters). Typically each tool 108a (108b) has an associated stocker 102a (102b) for wafer cassettes within the FOUPs 104, holding wafer lots waiting to be processed by the associated tool 108a (108b). FOUPs 104 are transferred from a load port 103 of a stocker 102a to a tool 108a via an overhead transport system (OHT) 107 having transport vehicles 106a, 106b, in a sequential order according to lot orders communicated from a real time dispatching system (not shown). Lots not queued for processing within the associated tool 108a remain in a wafer cassette within an associated FOUP 104; the associated FOUP 104 is routed to vehicle 106a of the overhead transport system 107 for transporting the FOUP 104 to the stocker 102b associated with the next tool 108b to be visited, for further processing.
FIG. 1 shows the stockers 102a, 102b installed beside the tools 108a, 108b, respectively. In a typical operating sequence, the processed lot 104 in the load port 109 of tool 108a is removed from the load port 109 and transported by an OHT vehicle 106a to the stocker 102b at the next destination tool 108b. The OHT vehicle 106a moves in one direction along its track 105. The next lot which is going to be processed is transported by another OHT vehicle 106b to the load port 109 of the tool 108a, from the stocker 102a. It takes a comparatively long time to finish swapping FOUPs, in part because two different vehicles 106a, 106b are used to remove a first FOUP 104 from the load port 109 of tool 108a and to transport a second FOUP from the stocker 102a to the load port 109. There may be a delay of about two minutes between the visit by vehicle 106a and the visit by vehicle 106b. 
The long FOUP swap time results in reduced tool duty cycle and reduced tool productivity.